When talking about Duke Nukem 3D even people who enjoy it struggle to put into words why they like it so much. People who don't know will say it was having an action hero pastiche, others will say it was the "interactivity" as in being able to play biliards in one level. Duke 3D actual biggest strength it's his level design, levels that are still to this day looking actual places while allowing gamey stuff to happen with small narratives and setpieces in them, no memorable story obv but levels leading into one another at the exit rather than just dropping you there and amusing theming in the levels adds a lot to it. Duke 3D actual loop is also pretty good and standout for the genre, with the core loadout from the first episode being usual but slightly different to further additions like the Shrinker and Devastator being nice and fuctional gimmicks, sadly there's also the Freezethrower and a good chunk of actually useless weapons and items, not many games are designed around the fact that the player could have a jetpack on themselves at all times though.

Episode 1: The most iconic the game gets and probably a good reason why the saying that the freeware episode in a retroshooter is the best one is said to begin with. This is as tight as the game gets not the other episodes aren't to look forward too.
Episode 2: Drop down in quality but not as severe as a lot of people make it out ot be. Levels are still really good but plagued with the overuse of suicide drones and less varied in their colors.
Episode 3: Back to earth with more colorful and better levels.
Episode 4: Pretty nutty. There are a lot of annoying encounters but over this is Duke 3D at it's limits.

Duke Nukem 3D is still to this day a standout in the genre with it's design and unique gimmicks. And it's still probably the weakest in the Build Engine Trinity.